Caliper sensor and method using mid-infrared interferometry

ABSTRACT

Non-contacting caliper measurements of free standing sheets such as porous polymer and paper detect mid-IR interferometric fringes created by the reflection of light from the top and bottom surfaces of the sheet. The technique includes directing a laser beam at a selected angle of incidence onto a single spot on the exposed outer surface wherein the laser beam comprises radiation having a wavelength in the 3-50 micron range and scanning the laser beam through a selected angle range as the laser beam is directed onto the exposed outer surface and measuring the intensity of an interference pattern that forms from the superposition of radiation that is reflected from the exposed outer surface and from the inner surface. Thickness can be extracted from the fringe separation in the interference pattern. Rotating and focusing elements ensure that the spot position on the sheet remains the same while varying the incident angle.

REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 14/103,733 which was filed on Dec. 11, 2013.

FIELD OF THE INVENTION

The present invention relates generally to interferometry techniques fornon-contacting thickness or caliper measurements of a moving sheet suchas porous polymer and paper and more particularly to methods ofdetecting mid-IR interferometric fringes created by the reflection oflight from the top and bottom surfaces of the sheet and thereafterextracting the thickness from the fringe separation.

BACKGROUND OF THE INVENTION

Caliper is one of the most important quality specifications of paper andplastic products. Traditional commercial on-line caliper measurementrequires the measuring heads to physically touch the web. Contacting theweb causes a number of issues with the two most significant ones beingthe marking of the sheet and the accumulating of dirt on the measuringheads, which leads to measurement drift and inaccuracy. More advancedtechniques make use of laser triangulation or confocal microscopytechniques but they still require a measuring head to contact one sideof the web. Moreover, prior art optical techniques are not suitable toall paper products because they are very sensitive to the scatteringproperties of the sheet. In addition, achieving better than 1 micronaccuracy is a challenge as these techniques rely on the differencebetween two independent distance measurements. As such, bothmeasurements must be stable with respect to each other in order toattain the required profile accuracy. This is difficult to achieve inthe paper scanner environment where the measurement heads are exposed tofrequent temperature changes and the positions of the paper and headsare subject to constant fluctuations. The art is desirous of developingreliable on-line techniques for accurately measuring the thickness webmaterials during production.

SUMMARY OF THE INVENTION

The present invention is based in part on the demonstration that mid-IRinterferometry is particularly effective in measuring web thickness. Inone aspect, the invention is directed to a method of measuring thethickness of a web, which has a first side and a second side, thatincludes the steps of:

supporting the web so that the web has a free standing portion where theweb has an exposed outer surface on the first side and an inner surfaceon the second side;

directing a laser beam at a selected angle of incidence onto a singlespot on the exposed outer surface wherein the laser beam comprisesradiation having a wavelength typically in the 3-50 micron andpreferably in the 8-25 micron range;

scanning the laser beam through a selected angle range as the laser beamis directed onto the spot on the exposed outer surface;

measuring the intensity of an interference pattern that forms from thesuperposition of radiation that is reflected from the exposed outersurface and from the inner surface; and

extracting the thickness of the web from the fringe separation in theinterference pattern. Preferred extraction techniques include regressionanalysis by least-squares fitting of the interference pattern intensitywith laser beam angle to an established relationship by using webthickness as the variable parameter. Another technique measures theoccurrence of interference minima.

A preferred technique of obtaining the thickness is by fitting theinterference pattern to the formula given by the relationship,I=A cos(δ)  (1)

where I is the measured intensity, A is the amplitude of theinterference pattern and δ is the phase difference between the radiationreflected from the outer surface and the radiation reflected from theinner surface. The phase difference δ is

$\begin{matrix}{\delta = {{\frac{4\pi\; d}{\lambda_{0}}\sqrt{\left( {n_{2}^{2} - {n_{1}^{2}\sin^{2}\theta_{1}}} \right)}} - \pi}} & (2)\end{matrix}$

The phase difference is expressed in terms of incident angle (θ₁),wavelength (λ₀), index of refraction of the air (n₁), index ofrefraction of the web (n₂) and web thickness (d), wherein angle,wavelength and indices are known, and web thickness is taken as avariable parameter, such as by finding the least-square error byadjusting a variable, which is the thickness.

In another aspect, the invention is directed to a non-contacting calipersensor, for measuring the thickness of a sheet of scattering materialhaving a first side and a second side, that includes:

a laser that provides a beam of incident radiation;

means for directing the incident radiation toward a single spot on anexposed outer surface on an exposed surface on the first side of thesheet wherein the incident radiation reaches the exposed surface at anangle of incidence of from 0 to 60 degrees;

means for detecting an interference pattern which forms by interferencebetween first radiation reflected from the exposed outer surface andsecond radiation reflected from an inner surface of the second side; and

means for analyzing the interference pattern to calculate the thicknessof the sheet.

In a preferred embodiment, radiation in the mid-infrared wavelength(3-50 microns), which is preferably in the 8-25 micron range, isdirected into the paper web and interferometric fringes created by thereflection of the light at the top and bottom surfaces of the web arerecorded. In comparison with radiation of shorter wavelengths, mid-IRwavelengths are less affected by scattering in the paper which makes theinventive technique suitable to applications unsuitable to prior arttechniques. Web thicknesses in the range of 20 microns to 2-3 mm can bemeasured if the caliper sensor wavelength is extended to the far-IR(typically having a wavelength of 50 microns to 1 mm) or terahertz range(typically having a wavelength of 100 microns to 1 mm). The web does notcome into contact with the measurement head in which the caliper sensoris positioned. The measurement can be performed in a reflection geometryrequiring only one measurement head.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a beam directed at a web and the scatter of the beamby the top and lower surfaces of the web;

FIGS. 2 to 6 show different embodiments of the caliper sensor;

FIG. 7 shows a sheet making system implementing a single-sided calipersensor in a dual head scanner;

FIG. 8 is a diagram of a system employing process measurements tocalculate the caliper of the web; and

FIG. 9 is a graph of intensity vs. angle illustrating fringeinterference signal for an 80 microns thick product at λ=15 microns andwith the index of refraction assumed to be 1.5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to non-contact sensor devices formeasuring the thickness of a film, web or sheet. While the sensor willbe illustrated in calculating the caliper of paper, it is understoodthat the sensor can measure thickness of a variety of flat materialsincluding, for example, coated materials, plastics, fabrics, and thelike. The sensor is particularly suited for thickness detection ofporous polymers (plastic) made of polyethylene, polypropylene,polyethylene terephthalate, polytetrafluoroethylene or polyvinylchloride.

FIG. 1 illustrates the electromagnetic radiation beam geometry incident,reflected and refracted on a web product 2 of thickness d and havingupper and lower sides or planes, plane 3 and plane 5, from which theincident electromagnetic radiation of wavelength λ₀ is reflected. Inaddition, the portion of the incident electromagnetic radiationpropagating into the web is refracted since the index of refraction isdifferent on different sides of plane 3. The distance between upper(plane 3) and lower (plane 5) sides is d. The index of refraction of theair around the web is n₁ and the index of refraction within the web isn₂. The optical path length difference between beam 7 and beam 9 isΔ=2n₂d cos θ₂. The corresponding phase difference is δ=k₀Δ−π, wherek₀=2π/λ₀. Interference minima occur atδ=(2m+1)π, where m=0,1,2, . . .  (3)

For instance, assuming that the mean incident angle is 45°, thewavelength of a laser light used is 15 μm, the web thickness is 80 μmand the index of refraction is 1.5, a range of ±7° in incident angle isrequired to measure 1 period of the interference.

In operation, once the interference pattern is obtained, standardtechniques can be implemented to ascertain the web thickness. One methodof extracting the material thickness and index of refraction from thespectra is to fit the angular spectra using the interferencerelationship given in equation 1 above. The thickness d and index n₂ canbe extracted from the fit. Another method is to record the angles of thezero crossings or interference minima which occur when equation 3 issatisfied. By plotting the values of sin² θ₁ at the zero crossing as afunction of m², a line of slope (λ₀/2dn₁)² and intercept (n₂/n₁)² areobtained. Web thickness, d, can be calculated. Assuming that n₁,typically air (n₁=1), is known then the index of refraction of thematerial n₂ can be calculated. The thickness is typically calculatedafter implicitly or explicitly calculating the index of refraction ofthe web.

The caliper sensor of the present invention preferably uses a quantumcascade laser (QCL) operating at a fixed wavelength in the 8-25 micronrange. A suitable QCL is commercially available from Daylight Solutions,Inc. (San Diego, Calif.). The laser beam is preferably directed at theweb being monitored at an angle in the range of 0 to 60 degrees and thespecular intensity is measured. FIG. 2 shows a caliper sensor thatincludes a stationary QCL 12, a pair of turning mirrors 8, 10, a pair ofrelay mirrors 4, 6 and stationary detector 14 that are positioned on thesame side of moving web 2 which is supported by rollers 30, 36. Turningmirrors 8 and 10 are mounted to rotational mechanisms 16 and 18,respectively. In operation, QCL 12 generates a laser beam 1A that isdirected toward turning mirror 8, which is shown to be in a firstposition, so that reflected beam 1B is directed by relay mirror 4 onto astationary position on moving web 2. Reflected radiation 1B from web 2is directed into detector 14 by relay mirror 6 and turning mirror 10.Detector 14 can comprise a photodiode that measures the intensity of theradiation captured. Each of the relay mirrors is preferably astationary, single conventional concave spherical mirror. Subsequently,turning mirrors 8 and 10 are rotated to their respective secondpositions so that incident radiation reaches the web at a differentangle than that of the initial beam 1A. The scanning process continuesuntil the entire range covered. Suitable detectors include, for example,a HdCdTe (mercury cadmium telluride) solid state detector.

FIG. 3 illustrates another configuration of the caliper sensor thatincludes a QCL 28, turning mirrors 20, 22, 24 and 26, and detector 31that are positioned on the same side of moving web 2. Each turningmirror is mounted to a rotational mechanism, which can be the sameconfiguration as that shown in FIG. 2. The orientations of the fourturning mirrors are coordinated so as to permit radiation from QCL 28 tobe scanned onto a stationary position on web 2 over a predeterminedangle range. In a preferred embodiment as shown in FIG. 2, turningmirror pairs 24 and 26 are arranged symmetrically and similarly turningmirror pairs 20 and 22 are arranged symmetrically. In this fashion, themirrors in each pair are rotated through the same angles.

FIG. 4 represents another configuration of the caliper sensor thatincludes quantum cascade laser 44 with associated conditioning optics 40and detector 46 with associated conditioning optics 42. The conditioningoptics 40, comprising a focusing lens 32 and a prism 48, is mounted on arotational mechanism that generates encoder signals, and allows changesto and determination of the incident angle on the web 2. Optionally, thefocusing lens and prism are mounted on a translation stage for signaloptimization. Similarly, conditioning optics 42 has a focusing lens 34and a prism 56 that allow signal optimization at the detector 46. Inoperation, QCL 44 generates a laser beam that is directed onto astationary position on web 2 at an initial incident angle throughconditioning optics 40. Synchronized movement of both prisms inconditioning optics 40 and 42 allows scanning of the radiation beam fromQCL 44 over a desired range of incident angle while maintaining the spotposition onto the web and maximizing signal at detector 46. For example,a 2 inch polyethylene cube with a 8 inch polyethylene focusing lens inconditioning optics 40 and 42 will give a 7 degree variation on thepredetermined initial incident angle on the web 2. The lenses and prismsof conditioning optics 40 and 42 preferred material is polyethylenebecause of the high transmission range bandwidth from 16-2500 um, butcould be made of Zinc Selenide (ZnSe), Silicon (Si), ThalliumBromide/Iodide (KRS-5) or Caesium Iodide (CsI) which are all good in theinfrared and far-infrared range.

FIG. 5 depicts a caliper sensor structure that employs a detector array52, associated optics 53 (such as a lens or micro lens array) along witha QCL 54, rotatable turning mirror 57, and relay mirror 58. A preferreddetector array comprises a linear array of discrete photodiodesconfigured to measure the intensity of the reflected radiation from astationary position on web 2 that is reflected at different angleswithout moving the detector array or optics to focus the reflectedradiation into the detector array. In operation, radiation from QCL 54is directed by turning mirror 57 onto a stationary position on web 2 atan initial angle of incident and the resulting reflected radiation iscaptured by detector array 52. Subsequently, the angle of incidence ischanged by rotating the turning mirror to a second position and theresulting reflected radiation is captured by detector array 52. Thisprocess ensures the radiation from QCL 54 is scanned onto web 2 over thedesired range of incident angles.

FIG. 6 illustrates a caliper sensor structure that employs a QCL 64,rotatable turning mirror 66, relay mirror 60, focusing optics 62 anddetector 68. The focusing optics 62 focuses reflected radiation intodetector 68. More than one mirror, lens, or combination may be used.

FIG. 7 illustrates a scanning sensor system 70 whereby the sensor isincorporated into a dual head scanner 78 that measure the caliper ofsheet 76 during continuous production. Scanner 78 is supported by twotransverse beams 72, 74 on which are mounted upper and lower scanningheads 80, 82. The operative faces of the lower and upper scanner heads80, 82 define a measurement gap that accommodates sheet 76. In oneparticular implementation of the caliper sensor, both the QCL anddetector of the sensor are incorporated into scanner head 80, whichmoves repeatedly back and forth in the cross direction across the widthof sheet 76, which moves in the machine direction (MD), so that thethickness of the entire sheet may be measured.

When the sensor is operating in the reflective mode as illustrated inFIG. 2, both the radiation source and receiver are housed within upperscanner head 80. When operating in the transmissive mode, a radiationsource is positioned in the upper scanning head 80 while the radiationreceiver is positioned in the lower scanning head 82.

The movement of the dual scanner heads 80, 82 is synchronized withrespect to speed and direction so that they are aligned with each other.The radiation source produces an illumination (spot size) on the sheet76 as the sensor moves repeatedly back and forth in the CD across thewidth of the moving sheet 76, so that the thickness of the entire sheetcan be monitored. The caliper sensor of the present invention directs abeam of radiation at the same spot on a sheet while varying the incidentbeam angle around that spot or pivot. In this regard, the time scaleover which the angle is varied needs to be fast enough so that thelength viewed by the sensor (while a scanner head is moving) in thecross-direction direction is minimized. The scanning period is typicallybelow 100 ms and preferably around 10 ms. The rotating and focusingelements ensure that the spot position on the sheet stays the same whilevarying the incident angle.

FIG. 8 depicts a process for controlling the manufacture of paper orother porous membranes or similar webs by continuously measuring thecaliper of the web. Digitized signals representing the intensity of themeasured radiation reflected from the web as the range of incidentangles is scanned is generated by the signal conditioning and digitizingstage 90 and is employed by microprocesser 92 to calculate caliper 94signals which can control actuators upstream and/or downstream ofscanner system 70 (FIG. 7) to regulate production mechanisms in responseto the caliper measurements.

A particular feature of mid infrared radiation is that the longerwavelengths compared to visible or near infrared make it less sensitiveto scatter by the web surface irregularities or roughness. Furthermore,mid infrared wavelengths are of the same order of magnitude as thethickness of typical web products such as paper and plastic films. Thecombination of the two results in interference fringes with high enoughvisibility that they can be measured and analyzed. A radiationtransmission window through water exists at around a wavelength, λ₀ ofapproximately 22 microns. That is, the total amount of transmittedradiation detected at this wavelength is least sensitive to water. Thus,using radiation as this wavelength is particularly suited for inmeasuring the thickness of paper, especially paper having a thicknesstypically in the range of 10 microns to 200 microns, FIG. 9 illustratesthe expected fringe interference that is formed using the caliper sensorof the present invention. The web is 80 microns thick and has an indexof refraction of 1.5 using radiation with a wavelength of 15 microns.

The foregoing has described the principles, preferred embodiments andmodes of operation of the present invention. However, the inventionshould not be construed as being limited to the particular embodimentsdiscussed. Thus, the above-described embodiments should be regarded asillustrative rather than restrictive, and it should be appreciated thatvariations may be made in those embodiments by workers skilled in theart without departing from the scope of the present invention as definedby the following claims.

What is claimed is:
 1. A non-contacting caliper sensor for measuring thethickness of a moving web of scattering material having a first side anda second side, comprising: a. a substantially monochromatic laser thatprovides a beam of incident radiation that has a wavelength in the 3-50micron range; b. means for directing the incident radiation toward asingle spot on an exposed outer surface on the first side of the movingweb wherein the incident radiation reaches a fixed position on theexposed surface at an angle of incidence of from 0 to 60 degrees withrespect to the moving web surface normal; c. means for detecting theinterference pattern which forms by superposition of a first radiationreflected from the exposed outer surface and a second radiationreflected from an inner surface of the second side; and d. means foranalyzing the interference pattern to calculate the thickness of themoving web at the single spot.
 2. The non-contacting caliper sensor ofclaim 1 wherein the laser is a quantum cascade laser that is tuned toemit at a wavelength substantially transmitted by the moving web.
 3. Thenon-contacting caliper sensor of claim 1 wherein the means for directingthe incident radiation comprises a driven closed-loop rotating mirrorsystem that sets the angle of incidence.
 4. The non-contacting calipersensor of claim 3 wherein the means for directing the incident laserradiation comprises: a. an optically flat front-surface tiltable mirrorthat is mounted on a first rotational axis and adjusted so that itscenterline of the first rotational axis is coincident with a plane of areflective surface of the optically flat mirror and having a laser beamimpingement point that is coincident with the first rotational axis; andb. a fixed concave front-surface mirror with a figure of revolution suchthat a laser beam impinging on the optically flat front surface mirrorand relayed onto the fixed concave front-surface mirror is focused andimaged onto the first surface of the web.
 5. The non-contacting calipersensor of claim 4 wherein the means for detecting the interferencepattern comprises: a. a fixed concave front-surface mirror with a figureof revolution such that the interference pattern formed from reflectedlaser light from the moving web is focused and directed toward anoptically flat front surface movable mirror; and b. an optically flatfront surface movable mirror that is mounted on a second rotational axisand adjusted so that the centerline of the second rotational axis iscoincident with the reflective surface of the optically flat mirror andso that the interference pattern is directed and focused onto aninfrared radiation sensitive detector.
 6. The non-contacting calipersensor of claim 3 wherein the means for directing the incident laserradiation comprises: a. a first optically flat front-surface tiltablemirror that is mounted on a first rotational axis and adjusted so thatits centerline of the first rotational axis is coincident with a planeof a reflective surface of the first optically flat front-surfacetiltable mirror and having a laser beam impingement point that iscoincident with the first rotational axis; and b. a second opticallyflat front-surface tiltable mirror that is mounted on a secondrotational axis and adjusted so that its centerline of the secondrotational axis is coincident with a plane of the reflective surface ofthe second optically flat front-surface tiltable mirror.
 7. Thenon-contacting caliper sensor of claim 6 wherein the means for detectingcomprises: a. a third optically flat front-surface tiltable mirror thatis mounted on a third rotational axis and adjusted so that itscenterline of the third rotational axis is coincident with a plane of areflective surface of the third optically flat front-surface tiltablemirror and directs an image of the interference pattern; and b. a fourthoptically flat front-surface tiltable mirror that is mounted on a fourthrotational axis and adjusted so that its centerline of the fourthrotational axis is coincident with a plane of a reflective surface ofthe second optically flat front-surface tiltable mirror and that relaysthe image of the interference pattern from the third optically flatfront-surface tiltable mirror to an infrared radiation sensitivedetector.
 8. The non-contacting caliper sensor of claim 3 wherein themeans for directing radiation of the interference pattern comprises apair of facing concave front-surface mirrors with a figure of revolutionsuch that off-axis specularly reflected laser light forming theinterference pattern from the moving web is reflected and directedtoward an infrared radiation sensitive detector while the axial laserlight forming the interference pattern is transmitted directly towardthe infrared radiation sensitive detector without reflection.
 9. Thenon-contacting caliper sensor of claim 3 wherein the means for detectingthe interference pattern is received by an infrared radiation sensitivedetector array that spans the range of angles of the reflectedinterference pattern.
 10. The non-contacting caliper sensor of claim 3wherein the means for directing the incident radiation comprisesconditioning optics that are coupled to the quantum cascade laser andmounted on an electrically driven translation and rotation stage such asto illuminated a spot position on the outer side of the first surface ofthe moving web which is stationary while simultaneously positioned at adesired illumination angle and translated to hold the illuminated spotposition constant.
 11. The non-contacting caliper sensor of claim 10wherein the means for detecting the interference pattern comprises asecond set of conditioning optics that is mounted on a secondelectrically driven translation and rotation stage and controlled suchthat the illuminated spot position on the outer surface of the firstsurface of the moving web is maintained on an axis of the secondconditioning optics and the axis is maintained parallel to the beam axiswith the output being directed to an infrared radiation sensitivedetector.
 12. The non-contacting caliper sensor of claim 1 wherein themoving web has a thickness in the range of 10 microns to 200 microns.13. The non-contacting caliper sensor claim 1 wherein the moving webcomprises paper or plastic made of polyethylene, polypropylene,polyethylene terephthalate, polytetrafluoroethylene or polyvinylchloride.
 14. The non-contacting caliper sensor of claim 1 wherein themoving web comprises paper and the radiation has a wavelength of about22 microns.
 15. The non-contacting caliper sensor claim 1 wherein themoving web comprises porous plastic that is made of polyethylene,polypropylene, polyethylene terephthalate, polytetrafluoroethylene orpolyvinyl chloride.
 16. A non-contacting caliper sensor for measuringthe thickness of a moving web of scattering material having a first sideand a second side, comprising: a. a quantum cascade laser that providesa beam of incident radiation that has a wavelength in the 3-50 micronrange; b. means for directing the incident radiation toward a singlespot on an exposed outer surface on the first side of the moving webwherein the incident radiation reaches a fixed position on the exposedsurface at an angle of incidence of from 0 to 60 degrees with respect tothe moving web surface normal; c. means for detecting the interferencepattern which forms by superposition of a first radiation reflected fromthe exposed outer surface and a second radiation reflected from an innersurface of the second side; and d. means for calculating the thicknessof the moving web at the single spot by utilizing a relationship amongthe laser beam incident angle, wavelength, index of refraction of theweb, and web thickness.
 17. The non-contacting caliper sensor of claim16 wherein the means for calculating the thickness is configured toanalyze the interference pattern to calculate the thickness of themoving web.
 18. The non-contacting caliper sensor of claim 17 whereinthe means for calculating the thickness is configured to applyleast-squares fitting of the interference pattern intensity distributionwith laser beam incident angle to a mathematical relationship by usingweb thickness as a fitting parameter.
 19. The non-contacting calipersensor of claim 16 wherein the means for calculating the thickness isconfigured to measure the occurrence of the interference minima.
 20. Thenon-contacting caliper sensor of claim 16 wherein the moving web has athickness in the range of 10 microns to 200 microns and wherein themoving web comprises paper or plastic made of polyethylene,polypropylene, polyethylene terephthalate, polytetrafluoroethylene orpolyvinyl chloride.